LONDON—British officials tore into technology companies for what they said was profiting from terrorist propaganda and offering secret places to communicate, in strong criticism of Silicon Valley days after a terrorist rampage outside Parliament left four people dead.

A U.K. intelligence official said investigators believe after interviews with people connected with Khalid Masood that he acted alone. Police are combing through...

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Police Investigate London Attacker Khalid Masood’s Motivation

Police are trying to determine whether Khalid Masood, the 52-year-old man who carried out the deadly rampage outside Parliament, acted on his own or had accomplices. Image: AP